Hypodermic needles are widely used in delivering and withdrawing fluids in medical practice. As originally used, hypodermic needles were used many times, the needles being sterilized between usages. A practitioner would sharpen the needles when they became dull, and then sterilize them prior to the next usage. Since the needles were reused, and often may have needed sharpening, the presence or absence of any lubrication on the outer surface of the needle had little effect on the penetration force or the pain perceived by the patient who was the recipient of the needle. With the development of commercially manufactured disposable needles that always have a fresh well-sharpened point, there was recognition that lubrication of the needle substantially reduced the pain perceived by the patient when a needle was administered to them.
A convention is followed in this disclosure wherein the portion of a device toward the practitioner is termed proximal and the portion of the device toward the patient is termed distal.
A tissue penetration by a hypodermic needle involves a sequence of events that collectively are perceived by the patient as whether or not the penetration caused pain. A distal point of the needle first touches the skin surface, stretches it, the point then cuts into the surface and begins penetration into the tissue. As the shaft of the needle passes through the original cut and into the tissue, there is also sliding friction of the tissue against the needle surface. In the hypodermic needle art when the forces for performing a hypodermic needle penetration are measured, the force measured prior to the needle point cutting the tissue is termed the "peak penetration force", also called "F2" and the force required to continue the penetration into the tissue is called the "drag force" or "F4". One primary component of the drag force is the sliding friction of the tissue against the surface of the needle shaft. When a subcutaneous or intramuscular penetration is made with a hypodermic needle, the penetration depth is generally between about 0.5 cm to about 2.5 cm into the patient's tissue. As a result, the practitioner generally does not generally perceive differences in the needle point's penetration of layers. Additionally, most subcutaneous and intra-muscular hypodermic penetrations are made at a relatively high rate (20 cm to 25 cm per second) and utilize the full length of the needle. The rapid penetration rate additionally reduces any perception of layers. The use of lubricant on the surface of hypodermic needles in combination with very well sharpened needles also significantly reduces both the peak penetration force and the drag force. When the reductions by lubrication of the peak penetration force and the drag force of the needle are coupled with the short duration resultant from the high penetration rate, a patient's perception of the painfulness of the penetration is generally significantly reduced. As a result, almost all single-use sterile disposable needles are supplied with a lubricant already applied to substantially the entire needle outside surface.